Process of casting nickel base alloys using water-soluble calcia cores

ABSTRACT

A process for producing heat-resistant metal articles from nickel base or cobalt base superalloys having at least one cavity therein comprising fabricating a ceramic core to the configuration of the cavity, the core consisting essentially of at least 50 percent calcia, the balance, if any, being a refractory ceramic material, the whole fired at a density range of 70 to 100 percent of theoretical density, supporting the core in a mold, casting the nickel alloy therearound and dissolving the core with hot water.

United States Patent Hulse [54] PROCESS OF CASTING NICKEL BASE ALLOYSUSING WATER-SOLUBLE CALCIA CORES [72] Inventor: Charles 0. I-Iulse,Manchester, Conn.

[73] Assignee: United Aircraft Corporation, East Hartford, Conn.

[22] Filed: July 8, 1970 [21] Appl. No.: 53,291

[52] U.S.Cl. ..l64/l32, 164/369, 18/DIG. 12, 249/61, 164/138, 164/36,106/389 [51] Int. Cl ..B22d 29/00 [58] Field oiSearch ..l64/131, 132,369, 36, 35,\

164/138, 34; 18/DIG. 11, DIG. 12; 249/61 Feb. 22, 1972 [56] ReferencesCited UNITED STATES PATENTS 2,383,812 8/1945 Navias 164/35 3,407,864 10/1 968 Anderko 164/41 Primary Examiner.l. Spencer Overholser AssistantExaminer-V. K. Rising Attorney-John D. Del Ponti [57] ABSTRACT A processfor producing heat-resistant metal articles from nickel base or cobaltbase superalloys having at least one cavi- 6 Claims, N0 Drawings PROCESSOF CASTING NICKEL BASE ALLOYS USING WATER-SOLUBLE CALCIA CORESBACKGROUND OF THE INVENTION This invention relates to a method forproducing cast metal articles containing inner passages or cavities andmore particularly relates to a process for the formation of cavities incastings of nickel base and cobalt base superalloys adapted for use ingas turbine engines for jet aircraft.

lt is known to fabricate gas turbine blades or vanes with cavities orpassages for cooling purposes. With the continuing requirement forhigher jet engine temperatures, the extent and complexity of theinternal passages is evergrowing and has presented severe problems whichcannot readily be overcome by present fabrication techniques. Atpresent, one .of the pri- 'mary methods for forming internal coolingpassages in these parts comprises casting the nickel or cobalt basealloy around a plurality of ceramic cores of zircon, zirconia, mulliteor the like with a binder of silica glass or pure fused silica andsubsequently removing them by long term exposure to caustic solutions atelevated temperatures in an autoclave. Unfortuconfiguration of thedesired cavity, supported in a conventional mold and the alloy is casttherearound. As will be appreciated, the nickel base and cobalt basealloys typically melt at about l,380 C. When casting, the melt isusually superheated an additional several hundred degrees. After thecast- 'ing is cooled, the core is then exposed and consequently dissolved in preferably hot or warm water. The calcia cores of the presentinvention have a relatively high water solubility at temperatures from50 to 200 C., with a greatly reduced solubility at lower temperatures.The rate of hydration ranges from 0.1 to 100 percent per minute on avolume basis. Investigation has indicated that satisfactory cores arethose which have a composition of 50 to 100 percent CaO, the balance, ifany, including a refractory ceramic inert to water such as MgO, CaO-Al OMgO-AhO and/or a refractory ceramic reactive with water such as SrO andBaO. The added refractory ceramic material must have a melting point ofat least l,800 C. and must be nonreactive with the molten nickel base orcobalt base superalloysunder consideration.

It is important to note that the strength of polycrystalline CaOcompares favorably to prior art materials which have been used for coresas shown in the table following.

TAB LE I nately, during removal of the cores from the casting, thecaustic solutions attack the cast alloy and thus make it moresusceptible to accelerated corrosion during use. This problem ofcorrosion is, of course, magnified by the increasing tem-- peratures ofthe engine environment, as indicated hereinbei fore. ln addition,strengthening of these core materials by firing, while desirable, issubstantially precluded since it causes diffusion of the silica binderand thus makes the removal problem substantially more difficult. Otherproblems, such as hot-tearing of the metal during cooling due to amismatch of coefficients of thermal expansion, have also indicated thereal need for a readily removable core for the nickel and cobalt basealloys which does not react with the alloy during casting and whichfurther, does not possess the shortcomings of the prior art.

SUMMARY OF THE INVENTION The present invention contemplates a processfor producing nickel and cobalt base metal castings containing cavitiestherein adapted for usage in a gas turbine engine which comprises thecasting of the article at temperatures of approximately l,600 C. orhigher which is to contain the cavity while using as a cavity former acore material which is nonreactive to the alloy during casting yet whichis water soluble for ready removal thereafter. It has been found that acalcia base ceramic core, consisting essentially of at least 50 percentCaO with the remainder, if any, including certain refractory ceramicsselected to achieve a predetermined solubility in water, will solve theproblems of the prior art.

DESCRIPTION OF THE PREFERRED EMBODIMENT and achieve the desireddensities. The core is formed to the 1 -11,000-extrap.

As will be noted, the thermal expansions of the materials are includedin the table and can be compared with that of nickel which is l7.l 1O C.over the same temperature range. It cam be seen that calcia has a highcoefficient of thermal expansion, approximating that of nickel. Thus,upon cooling, a

core of this material will shrink almost as quickly as the metal castingaround it, thereby minimizing hot tearing problems due to tensile forcesset up in the casting as it contracts down upon the core.

In order to successfully utilize the calcia base core, it has been foundthat a density of 70 to 100 percent of theoretical density must beprovided. This range applies to the core whether or not additives asmentioned hereinbefore are present. Within the range stipulated, themore dense bodies, i.e., those from approximately to percent may be useduncoated while the less dense bodies, i.e., those from 70 toapproximately 85 percent, are to be provided with a protective coatingto prevent hydration in air. Suitable protective coatings of imperviousinorganic material such as cellulose acetate, polystyrene or Krylon(87.66 percent volatilealiphatic, aromatic and halogenated hydrocarbons,9.18 percent nonvolatile-acrylic ester resin chlorinated rubber butyebenzyl phthalate.) are burned away during heat up of the mold before orduring casting,

The composition and density of the core selected depends upon a balanceof factors in view of the particular problems at hand for a specificapplication. In general, the more porous the core, the faster will beits removal by water since the water is able to more easily penetrateand react with the calcia. if the porosity results in too muchsensitivity to water vapor in the air or to water vapor present during astep in the mold forming process, the reactivity may be reduced byincluding an inert ceramic, as for example MgO, in the core batchcomposition.

Ordinarily, higher densities mean higher strengths. Where strength is aproblem with a particular core shape, a high density will be desirableor necessary. Unfortunately, a high-density core is much more slowlyremoved by water than a lower density one. In order to offset thisdisadvantage, an addition of SrO and/or BaO to increase reactivity withwater should be made. SrO and BaO are well suited to their task sincethey are both very refractory ceramic oxides, have the same crystalstructure as CaO and react much more rapidly than CaO to water.

The following examples are set forth for the purpose of illustration andnot limitation ofthe present invention.

EXAMPLE I Commercially available calcium carbonate was poured intocarbon dies having the curvilinear configuration of the air coolingpassages of a turbine blade, and was hot pressed in vacuum at I,400 C.and 5,000'lb./in. for 30 minutes. The calcia core formed in theforegoing manner was then placed in standard ceramic molds and a nickelbase alloy (nominal composition, by weight; 15 Cr, 15.3 Co, 4.4 M0, 3.4Ti, 4.3 A1, 0.02 B, Bal. Ni) was cast around it in a vacuum at atemperature of l,550 C. After casting, the cores were removed usingwater heated to a temperature of 1 C. Metallographic examination ofsectioned portions of the casting revealed that the core material didnot react with or corrode the cast metal.

EXAMPLE 1] Using the same technique as in Example 1, a calcia core wasfabricated having a porosity of percent. The core was placed in astandard ceramic mold and a cobalt base alloy (MAS5382-nominalcomposition by weight; 25.5 Cr, l0.5 Ni, 7.5 W, 2.0 Fe, 0.5 C, Bal. Co.)was cast around it in air at a temperature of l,600 C. It was quicklyremovable from the casting without reacting with or corroding same.

While particular embodiments have been described, it will be understoodthat various modifications may be made without departing from the scopeof this invention. Thus the soluble cores can be removed by means otherthan hot water, as for example heating under steam pressure in anautoclave with KOH or other additives to hasten the process. It willfurther be appreciated that while the description herein has beendirected primarily to cores, it is not intended to exclude usage of thecalcia base material in other modes, as for example an external castingmold.

What is claimed is:

l. A method for producing heat-resistant metal articles having at leastone cavity therein from a cast nickel or cobalt base superalloycomprising:

fabricating a ceramic core to the configuration of the cavity,

said ceramic core consisting essentially of at least 50 percent byweight CaO, the remainder, if any, being at least one refractory ceramicmaterial for controlling the water solubility of the core;

supporting said ceramic core in a mold;

casting the alloy around said ceramic core in the mold;

removing the casting from the mold; and

dissolving said ceramic core in water.

2. The method of claim 1 wherein the ceramic core is at a density rangeof 70 to percent of theoretical density.

3. The method of claim 2 wherein the ceramic core contains a refractoryceramic material inert to water.

4. The method of claim 3 wherein the refractory ceramic material isselected from the group consisting of MgO, ZrO CaO--Al O and MgO-Al O 5.The method of claim 2 wherein the ceramic core contains a refractoryceramic material reactive with water.

6. The method of claim 5 wherein the refractory material is selectedfrom the group consisting of SrO and BaO.

2. The method of claim 1 wherein the ceramic core is at a density rangeof 70 to 100 percent of theoretical density.
 3. The method of claim 2wherein the ceramic core contains a refractory ceramic material inert towater.
 4. The method of claim 3 wherein the refractory ceramic materialis selected from the group consisting of MgO, ZrO2, CaO-Al2O3 andMgO-Al2O3.
 5. The method of claim 2 wherein the ceramic core contains arefractory ceramic material reactive with water.
 6. The method of claim5 wherein the refractory material is selected from the group consistingof SrO and BaO.